Oxygen adsorption on γ-TiAl surfaces and the related surface phase diagrams: A density-functional theory study

We have constructed the surface phase diagrams for oxygen adsorption on γ-TiAl low-index surfaces using density-functional theory calculations. From these surface phase diagrams, the selective oxidation behaviors of the γ-TiAl surfaces and the corresponding polycrystalline systems can be easily understood and predicted. For the (1 0 0) surface, complete selective oxidation of titanium is favored and a titanium oxide layer may be produced at the initial stage of oxidation. For the (1 1 0) and (0 0 1) surfaces, only titanium oxides may form. For the γ-TiAl polycrystalline system, O may induce complete Ti and Al surface segregations on the Ti-rich and Al-rich conditions, respectively. In addition, the microscopic oxidation mechanisms are identified and the experimental results are successfully explained. More importantly, by comparing the different TiAl surface orientations, a comprehensive surface phase diagram is constructed to study the oxidation behaviors of polycrystalline γ-TiAl. This method can also be applied to other polycrystalline materials.